In milling certain types of powders it is often necessary or desirable to have a positive control of the atmosphere within the mill at all times. For example, readily oxidizable materials such as aluminum, titanium, magnesium, lithium and fine powders of many compositions are combustible or even explosive under certain conditions or they may be contaminated by the presence of air. In milling such materials the control of the atmosphere must extend to charging and discharging of the mill without opening the mill to air. The problem of exposure to air during operation of the mill, charging materials to the mill and unloading the mill, extends to sampling of powders in the mill, e.g. to determine the current state of the processing.
The present invention is not restricted to the processing of any particular powders or any particular grinding mill. It is especially useful, however, for sampling powders which must be processed in an airtight system and in a batch-type rotary mill. For that reason it is described below with reference to metal powders which are readily oxidized and are processed to obtain dispersion strengthened materials or alloys by powder metallurgy routes. Of necessity the milling of such materials must be carried out in a controlled atmosphere, e.g., sealed to air.
The problem encountered in milling powders are particularly troublesome in the mechanical alloying of readily oxidizable metals such as aluminum, magnesium and lithium. Mechanical alloying has been described in detail in the literature in patents. U.S. Pat. No. 3,740,210, U.S. Pat. No. 3,816,080 and U.S. Pat. No. 3,837,930, for example, involve the mechanical alloying of aluminum alloys and other composite materials containing aluminum. In the practice of mechanical alloying the components of the product are charged in powder form into a high energy milling device such as a ball mill where, in an environment free of or reduced in amount of free or combined oxygen, the powders which are dry or substantially dry are ground down to a very fine size initially, prior to particle agglomeration in the latter stages of the process. This initial grinding increases the total surface area of the metallic powders significantly. Since any freshly exposed surface of the powder is not oxidized, it is very hungry for oxygen to the extent that the powders in this condition will burn and/or might explode spontaneously if exposed to air. Thus, any port in the mill, for example, for charge, discharge or sampling of powders, is a source of potential danger from the standpoint of the quality of the product produced and the possibility of a fire and/or an explosion. To avoid the hazards of exposure to air, any discharge of material must be effected while maintaining a positive control of the environment in the mill, and in the discharge and/or sampling system.
It has been known to operate a rotary ball mill with a plug in an opening in the shell, the plug being replaceable with a grate during discharge. For protection of the environment during discharge the shell is enclosed in a housing. When the milling cycle is finished the housing is opened to replace the plug with a grate, then the housing is closed for the discharge cycle. During the discharge cycle the discharge opening is rotated to the underside of the shell, thereby permitting the powder to run out into the housing. The rotation for discharge of material can be repeated. This arrangement is not satisfactory. It opens the system to the atmosphere when the plug is replaced by the grate. Powder discharged from the shell tends to accumulate in the housing, thereby requiring cleaning out of the housing after each run and further opening the system to air. Opening of the housing is a source of contamination of the powder discharged from the mill and to subsequent runs in the mill. A further serious problem is that when the shell rotates inside the housing the discharging powder may be in the explosion range in terms of concentration of various portions of powder discharged in any cycle. Another proposed method for discharge is by gas sweep through the mill to pick up particles and carry them to a classification system. This involves the use of a combination of devices such as dropout chambers, cyclones, bag filters, blowers and the like. Since the powder conveyed is combustible and/or explosive, this gas sweep system poses a significant hazard. Furthermore, it is difficult to seal against infiltration of air and against leaks.
The conventional discharge devices are designed to discharge large amounts of powder from the mill. However, it is often highly desirable to check the progress of powder in the mill by obtaining just small samples. The same problems of contamination, burning and explosions apply to obtaining small samples as they do to discharging the mill. It is necessary to obtain samples of powders from the mill without exposing the mill contents or the samples to air. The present invention is directed to obtaining samples from grinding mills without exposing the mill contents or the samples to air. It is an advantage of the present system that it can be incorporated into existing mills.
In co-pending U.S. patent application Ser. No. 712,570 filed of even date herewith a system for charging a ball mill under seal is disclosed. The present invention, while not limited to the disclosed system, can be readily adapted for use with it.